U.S. patent application number 16/067998 was filed with the patent office on 2019-01-10 for double-sided imaging light guide with embedded dichroic filters.
The applicant listed for this patent is Vuzix Corporation. Invention is credited to Robert J. SCHULTZ, Paul J. TRAVERS.
Application Number | 20190011708 16/067998 |
Document ID | / |
Family ID | 59274191 |
Filed Date | 2019-01-10 |
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United States Patent
Application |
20190011708 |
Kind Code |
A1 |
SCHULTZ; Robert J. ; et
al. |
January 10, 2019 |
DOUBLE-SIDED IMAGING LIGHT GUIDE WITH EMBEDDED DICHROIC FILTERS
Abstract
An imaging light guide has a waveguide formed as a coated
substrate having first and second surface coatings. A first
in-coupling diffractive optic on the first coating directs
diffracted light of a first wavelength range into the waveguide
along a first direction. A second in-coupling diffractive optic on
the second coating directs diffracted light of a second wavelength
range into the waveguide along a second different direction. A
first dichroic patch between the first surface of the substrate and
the first surface coating for (a) transmitting the first wavelength
range, (b) transmitting the second wavelength range through a range
of incidence angles, and (c) reflecting the second wavelength range
through a higher range of incidence angles. A second dichroic patch
between the second surface of the substrate and the second surface
coating for transmitting the second wavelength range and reflecting
the first wavelength range.
Inventors: |
SCHULTZ; Robert J.; (Victor,
NY) ; TRAVERS; Paul J.; (Honeoye Falls, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vuzix Corporation |
West Henrietta |
NY |
US |
|
|
Family ID: |
59274191 |
Appl. No.: |
16/067998 |
Filed: |
January 5, 2017 |
PCT Filed: |
January 5, 2017 |
PCT NO: |
PCT/US2017/012332 |
371 Date: |
July 3, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62275557 |
Jan 6, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/0026 20130101;
G02B 2027/0116 20130101; G02B 2027/0123 20130101; G02B 2027/0178
20130101; G02B 27/44 20130101; G02B 6/34 20130101; G02B 5/32
20130101; G02B 27/0172 20130101; G02B 6/0016 20130101; G02B
2005/1804 20130101; G02B 2027/0114 20130101 |
International
Class: |
G02B 27/01 20060101
G02B027/01; F21V 8/00 20060101 F21V008/00 |
Claims
1. An imaging light guide comprising: a waveguide formed as a
substrate having a first surface with a first surface coating and a
second surface, opposite the first surface and having a second
surface coating; a first in-coupling diffractive optic formed on
the first surface coating and oriented to direct a first beam of
diffracted light of a first wavelength range into the waveguide
along a first direction; a second in-coupling diffractive optic
formed on the second surface coating and oriented to direct a
second beam of diffracted light of a second wavelength range into
the waveguide along a second different direction; a first dichroic
patch located between the first surface of the substrate and the
first surface coating and formed to: (i) transmit light of the
first wavelength range, (ii) transmit light of the second
wavelength range through a limited range of incidence angles, and
(iii) reflect light of the second wavelength range through a higher
range of incidence angles; and a second dichroic patch located
between the second surface of the substrate and the second surface
coating and formed to transmit light of the second wavelength range
and reflect light of the first wavelength range.
2. The imaging light guide of claim 1 wherein the first dichroic
patch forms a high wavelength pass filter and the second dichroic
patch forms a low wavelength pass filter.
3. The imaging light guide of claim 1 further comprising: a first
turning diffractive optic along a path in the first direction and
disposed to redirect the first beam of diffracted light toward a
first out-coupling diffractive optic. a second turning diffractive
optic along a path in the second direction and disposed to redirect
the second beam of diffracted light toward a second out-coupling
diffractive optic.
4. The imaging light guide of claim 3 wherein the first in-coupling
diffractive optic and the second in-coupling diffractive optic are
aligned along a common normal to the first and second surfaces of
the substrate, and wherein the first turning diffractive optic and
the second turning diffractive optic are not aligned along a common
normal to the first and second surfaces of the substrate.
5. The imaging light guide of claim 4 wherein the first
out-coupling diffractive optic and the second out-coupling
diffractive optic are aligned along a common normal to the first
and second surfaces of the substrate.
6. The imaging light guide of claim 1 wherein the substrate is an
optical glass or plastic and the first surface coating is a
polymer.
7. The imaging light guide of claim 1 wherein the first in-coupling
diffractive optic is a volume hologram.
8. The imaging light guide of claim 1 wherein the first in-coupling
diffractive optic is a diffraction grating.
9. The imaging light guide of claim 1 in which the substrate is a
planar substrate.
10. The imaging light guide of claim 1 in which the first dichroic
patch transmits light of the second wavelength range at incidence
angles less than about 15 degrees.
11. The imaging light guide of claim 1 in which the first dichroic
patch reflects light of the second wavelength range at incidence
angles that supports total internal reflection along the
waveguide.
12. The imaging light guide of claim 1 further comprising: a first
turning diffractive optic in the path of the first direction and
disposed to redirect the first beam of diffracted light toward a
first out-coupling diffractive optic, wherein both the first
out-coupling diffractive optic and the first turning diffractive
optic are formed in the first surface coating; and a second turning
diffractive optic in the path of the second direction and disposed
to redirect the second beam of diffracted light toward a second
out-coupling diffractive optic, wherein both the second
out-coupling diffractive optic and the second turning diffractive
optic are formed in the second surface coating.
Description
TECHNICAL FIELD
[0001] This invention generally relates to optical light guides for
conveying image-bearing light in multiple color channels to a
viewer particularly for use in video eyewear or augmented or
virtual reality near-eye displays.
BACKGROUND OF THE INVENTION
[0002] Head-Mounted Displays (HMDs), which include near eye
displays in a form resembling conventional eyeglasses or
sunglasses, are being developed for a range of diverse uses,
including military, commercial, industrial, fire-fighting, and
entertainment applications. For many of these applications, there
is particular value in forming a virtual image that can be visually
superimposed over the real-world image that lies in the field of
view of the HMD user. Light guides incorporating various types of
waveguides, relay image-bearing light to a viewer in a narrow
space, acting as exit-pupil expanders for redirecting the virtual
image to the viewer's pupil and enabling this superposition
function.
[0003] In the conventional light guide, collimated angularly
related light beams from an image source are coupled into the light
guide substrate, generally referred to as a waveguide, by an input
optical coupling such as an in-coupling diffraction grating, which
can be formed on a surface of the substrate or buried within the
substrate. Other types of diffractive optics could be used as input
couplings, including diffractive structures formed of alternating
materials of variable index such as holographic polymer dispersed
liquid crystal (HPDLC) or volume holograms. The diffractive optics
could also be formed as surface relief diffraction gratings. The
collimated light beams can be directed out of the waveguide by a
similar output optical coupling, which can also take the form of a
diffractive optic. The collimated angularly related beams ejected
from the waveguide overlap at an eye relief distance from the
waveguide forming an exit pupil within which a virtual image
generated by the image source can be viewed. The area of the exit
pupil through which the virtual image can be viewed at the eye
relief distance is referred to as an "eyebox."
[0004] The output coupling can also be arranged for enlarging the
exit pupil. For example, the collimated beams can be enlarged in
one dimension by offsetting partially reflected portions of the
collimated beams in a direction at which the collimated beams
propagate along the output coupling or by ejecting collimated beams
of different angles from different positions along the waveguide to
more efficiently overlap the collimated beams at the eye relief
distance from the waveguide.
[0005] A so-called "turning optic" located along the waveguide
between the input coupling and the output coupling, can be used for
expanding pupil size in a second dimension. The expansion can be
effected by offsetting reflected portions of the collimated beam to
enlarge a second dimension of the beams themselves or by directing
the collimated beams to different areas of the output coupling so
the collimated beams of different angles are ejected from different
positions to more efficiently overlap within the eyebox. The
turning optic can also take the form of a diffractive optic and,
especially when located between the diffraction gratings of the
input coupling and output coupling, can also be referred to as an
intermediate grating.
[0006] Although conventional light guide mechanisms have provided a
significant reduction in bulk, weight, and overall cost of display
optics, there are still issues to resolve. Suitable separation of
color channels is need in order to prevent cross-talk, in which
color is processed and displayed from the wrong color channel.
Cross-talk can lead to disparity between the color image data and
the displayed color, and can also be a cause of objectionable color
shifts, perceptible across the image field. Attempts to correct
this problem have included stacking approaches in which multiple
waveguides are stacked together with optional filters to prevent
color from being directed to the wrong channel. Stacking, however,
leads to thicker devices, adds weight, reduces brightness, and has
not provided highly satisfactory results.
[0007] Thus, it can be appreciated that there is a need for
improved designs that still provide the pupil expansion
capabilities of the optical light guide, but allow these devices to
be thinner and more lightweight, without compromising image quality
and color balance.
SUMMARY OF THE INVENTION
[0008] It is an object of the present disclosure to advance the art
of image presentation when using compact head-mounted devices and
similar imaging apparatus. Advantageously, embodiments of the
present disclosure provide an improved double-sided beam expander
capable of handling two color channels within a single thickness of
substrate.
[0009] These and other aspects, objects, features and advantages of
the present invention will be more clearly understood and
appreciated from a review of the following detailed description of
the preferred embodiments and appended claims, and by reference to
the accompanying drawings.
[0010] According to an aspect of the present disclosure, there is
provided an imaging light guide including a waveguide formed as a
substrate having a first surface with a first surface coating and a
second surface opposite the first surface and having a second
surface coating. A first input coupling (in-coupling) diffractive
optic is formed on the first surface coating and oriented to direct
a first beam of diffracted light of a first wavelength range into
the waveguide in a first direction. A second input coupling
(in-coupling) diffractive optic is formed on the second surface
coating and oriented to direct a second beam of diffracted light of
a second wavelength range into the waveguide in a second different
direction. A first dichroic patch is located between the first
surface of the substrate and the first surface coating and is
formed to (i) transmit light of the first wavelength range, (ii)
transmit light of the second wavelength range through a limited
range of incidence angles, and (iii) reflect light of the second
wavelength range through a higher range of incidence angles. A
second dichroic patch is located between the second surface of the
substrate and the second surface coating and is formed to transmit
light of the second wavelength range and to reflect light of the
first wavelength range.
[0011] The first dichroic patch preferably transmits light of the
second wavelength range at incidence angles centered about zero
degrees, preferably less than about 15 degrees, and reflects light
of the second wavelength range at incidence angles in a higher
range, preferably greater than 40 degrees, that supports total
internal reflection along the waveguide.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0012] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings.
[0013] FIG. 1 is a schematic diagram that shows a simplified
cross-sectional view of one possible configuration of a light guide
arranged as waveguide incorporating a monocular type diffractive
beam expander.
[0014] FIG. 2 is a perspective view that shows a light guide
arranged as waveguide incorporating a diffractive beam expander
including a turning grating.
[0015] FIG. 3A is a perspective view that shows a light guide
arranged as waveguide incorporating a double-sided diffractive beam
expander.
[0016] FIG. 3B is an exploded view of the FIG. 3A embodiment
showing the distribution of components for two different color
channels on front and back surfaces of the waveguide.
[0017] FIG. 4A is a side view that shows the input end of the light
guide with opposing in-coupling diffractive optics for a
double-sided diffractive beam expander. Respective grating
orientations are illustrated in a plan view.
[0018] FIG. 4B is a perspective view that shows the relative
rotational orientations of the in-coupling diffractive optics for
the two different color channels and their corresponding grating
vectors.
[0019] FIG. 5 is a top view that shows a lay out of components of a
double-sided light guide according to one approach for maintaining
a high rotational angle to separate color channels.
[0020] FIG. 6 is a graph that shows the relationship of rotational
angle to diffraction efficiency for light moving through the
waveguide at oblique angles.
[0021] FIG. 7 is a side view that shows the input end of the light
guide with opposing in-coupling diffractive optics for a
double-sided diffractive beam expander also showing the effects of
unintended diffraction of a beam by the opposite color channel at
this point.
[0022] FIG. 8A is a perspective exploded view that shows a light
guide formed as a multilayer structure.
[0023] FIG. 8B is a cross-sectional side view that shows the light
guide formed as a multilayer structure.
[0024] FIG. 9A is a side view that shows the input end of the light
guide managing a red color channel by dichroic patches.
[0025] FIG. 9B is graph showing a characteristic transmittance
curve that shows the behavior of one dichroic coating of the light
guide.
[0026] FIG. 9C is a graph showing a characteristic transmittance
curve that shows the behavior of the other dichroic coating of the
imaging light guide.
[0027] FIG. 10 is a side view that shows the input end of the light
guide managing a blue-green color channel by dichroic patches.
[0028] FIG. 11 is a side view that shows the input end of the light
guide managing the combined behaviors of the input couplings and
dichroic coatings for both color channels.
[0029] FIG. 12 is a perspective view that shows a display system
for augmented reality viewing using imaging light guides of the
present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present description is directed in particular to
elements forming part of, or cooperating more directly with,
apparatus in accordance with the invention. It is to be understood
that elements not specifically shown or described may take various
forms well known to those skilled in the art.
[0031] Where they are used herein, the terms "first", "second", and
so on, do not necessarily denote any ordinal, sequential, or
priority relation, but are simply used to more clearly distinguish
one element or set of elements from another, unless specified
otherwise. The terms "top" and "bottom" do not necessarily
designate spatial position but provide relative information about a
structure, such as to distinguish opposing surfaces of a planar
(flat) waveguide.
[0032] In the context of the present disclosure, the terms
"viewer", "operator", "observer", and "user" are considered to be
equivalent and refer to the person who wears the HMD viewing
device.
[0033] As used herein, the term "energizable" relates to a device
or set of components that perform an indicated function upon
receiving power and, optionally, upon receiving an enabling
signal.
[0034] The term "actuable" has its conventional meaning, relating
to a device or component that is capable of effecting an action in
response to a stimulus, such as in response to an electrical
signal, for example.
[0035] The term "set", as used herein, refers to a non-empty set,
as the concept of a collection of elements or members of a set is
widely understood in elementary mathematics. The term "subset",
unless otherwise explicitly stated, is used herein to refer to a
non-empty proper subset, that is, to a subset of the larger set,
having one or more members. For a set S, a subset may comprise the
complete set S. A "proper subset" of set S, however, is strictly
contained in set S and excludes at least one member of set S.
[0036] In the context of the present disclosure, the term "oblique"
means at an angle that is not an integer multiple of 90 degrees.
Two lines, linear structures, or planes, for example, are
considered to be oblique with respect to each other if they diverge
from or converge toward each other at an angle that is at least
about 5 degrees or more away from parallel, or at least about 5
degrees or more away from orthogonal.
[0037] In the context of the present disclosure, the terms
"wavelength band" and "wavelength range" are equivalent and have
their standard connotation as used by those skilled in the art of
color imaging and refer to a range of light wavelengths that are
used to form one or more colors in polychromatic images. Different
wavelength bands are directed through different color channels,
such as to provide red, green, and blue primary colors in
conventional color imaging applications.
[0038] As an alternative to real image projection, an optical
system can produce a virtual image display. In contrast to methods
for forming a real image, a virtual image is not formed on a
display surface. That is, if a display surface were positioned at
the perceived location of a virtual image, no image would be formed
on that surface. A virtual image display has a number of inherent
advantages for an augmented reality display. For example, the
apparent size of a virtual image is not limited by the size or
location of a display surface. Additionally, the source object for
a virtual image may be small; a magnifying glass, as a simple
example, provides a virtual image of its object. In comparison with
systems that project a real image, a more realistic viewing
experience can be provided by forming a virtual image that appears
to be some distance away. Providing a virtual image also obviates
any need to compensate for screen artifacts, as may be necessary
when projecting a real image.
[0039] In the context of the present disclosure, the term "coupled"
is intended to indicate a physical association, connection,
relation, or linking, between two or more components, such that the
disposition of one component affects the spatial disposition of a
component to which it is coupled. For mechanical coupling, two
components need not be in direct contact, but can be linked through
one or more intermediary components. A component for optical
coupling allows light energy to be input to, or output from, an
optical apparatus. The terms "beam expander" and "pupil expander"
are considered to be synonymous, used interchangeably herein.
[0040] FIG. 1 is a schematic diagram that shows a simplified
cross-sectional view of one conventional configuration of a light
guide 10 arranged as a monocular type light diffractive beam
expander or exit pupil expander comprising an input coupling
element such as an in-coupling diffractive optic 110, and an output
coupling element, such as an out-coupling diffractive optic 120
arranged on a transparent and planar waveguide 22 having a
substrate S. In this example, in-coupling diffractive optic 110 is
shown as a reflective type diffraction grating; however,
in-coupling diffractive optic 110 could alternately be a
transmissive diffraction grating, volume hologram or other
holographic diffraction element, or other type of optical component
that provides diffraction for the incoming, image-bearing light,
arranged on a lower surface 12 of the waveguide substrate S, where
the in-coming light wave WI first interacts with the waveguide
substrate S.
[0041] When used as a part of a virtual display system, in-coupling
diffractive optic 110 couples each of a plurality of angularly
related in-coming image-bearing light beams WI from an imager, via
suitable front end optics (not shown), into the substrate S the
waveguide 22. The input light beams WI are diffracted by
in-coupling diffractive optic 110. For example, first order
diffracted light propagates as an angularly related set of beams WG
along the substrate S, moving toward the right in the FIG. 1
system, toward out-coupling diffractive optic 120. Between gratings
or other types of diffractive optics, light is channeled or
directed along the waveguide 22 by Total Internal Reflection (TIR).
Out-coupling diffractive optic 120 contributes to beam expansion
via multiple diffractive encounters with the propagating light
beams WG along its length, i.e., along the x-axis in the view of
FIG. 1, and directs the diffracted light from each encounter
outwards towards the intended location of an observer's eye.
[0042] The perspective view of FIG. 2 shows an imaging light guide
20 arranged as a known beam expander that provides beam expansion
along x- and y-axes using an intermediate turning grating TG to
redirect the light output (first diffracted mode) from in-coupling
diffractive optic 110 to out-coupling diffractive optic 120. In the
FIG. 2 device, in-coupling diffractive optic 110 containing
periodic rulings with a period d diffracts angularly related
incoming input optical beams WI into the waveguide 22 as a set of
angularly related beams WG, propagating by total internal
reflection in an initial direction towards the intermediate turning
grating TG. Intermediate grating TG is termed a "turning grating"
because of its function in the optical path, redirecting the beams
WG from within the waveguide 22 according to its grating vector in
a direction towards the out-coupling diffractive optic 120, thereby
accounting for a difference in angle between the grating vectors of
the in-coupling diffraction optic 110 and the out-coupling
diffraction optic 120. Intermediate grating TG, which has angular
orientation of diffraction elements and a spacing geometry
determined by spacing period d, not only redirects the internally
reflected beams WG but also contributes to beam expansion via
multiple diffractive encounters with the light beams WG along the
initial direction of propagation, i.e., along the y-axis in the
view of FIG. 2. The out-coupling diffractive optic 120 contributes
to an orthogonal beam expansion via multiple diffractive encounters
with the light beams WG along the redirected direction of
propagation, i.e., along the x-axis in the view of FIG. 2.
[0043] The grating vectors, generally designated k and shown with
subscripts where they are specific to light within a color channel,
extend parallel to the plane of the waveguide surface and are in
the direction of the periodicity of the in-coupling and
out-coupling diffractive optics 110 and 120, respectively.
[0044] In considering a light guide design used for imaging it
should be noted that image-bearing light traveling within a
waveguide is effectively encoded by the in-coupling optics, whether
the in-coupling mechanism uses gratings, holograms, prisms,
mirrors, or some other mechanism. Any reflection, refraction,
and/or diffraction of light that takes place at the input must be
correspondingly decoded by the output in order to re-form the
virtual image that is presented to the viewer.
[0045] A turning grating TG, placed at an intermediate position
between the input and output couplings, such as the in-coupling and
out-coupling diffractive optics 110 and 120, is typically chosen to
minimize any changes on the encoded light. As such, the pitch of
the turning grating preferably matches the pitch of the in-coupling
and out-coupling diffractive optics 110 and 120. In addition, the
virtual image can be preserved by orienting the turning grating at
around 60 degrees to in-coupling and out-coupling diffractive
optics 110 and 120 in such a way that the encoded ray bundles are
turned 120 degrees by one of the 1st reflection orders of the
turning grating TG. The diffractive effects of the turning grating
TG are most pronounced on the vector component of the incoming rays
that are parallel to the grating vector of the turning grating.
Turning gratings so arranged redirect ray bundles within the guide
substrate while minimizing any changes to the encoded angular
information of the virtual image. The resultant virtual image in
such a designed system is not rotated. If such a system did
introduce any rotation to the virtual image, the rotational effects
could be non-uniformly distributed across different field angles
and wavelengths of light, thus causing unwanted distortions or
chromatic aberrations in the resultant virtual image.
[0046] The use of turning grating TG as envisioned for certain
embodiments described herein preserves an inherent geometrical
accuracy to the design of the light guide 20 so that the input beam
and output beam are symmetrically oriented with respect to each
other. With proper grating TG spacing and orientation, grating
vectors k direct the light from the in-coupling diffractive optic
110 to the out-coupling diffractive optic 120. It should be noted
that the image that is formed for the imaging light guide viewer is
a virtual image, focused at infinity or at least well in front of
the light guide 20, but with the relative orientation of output
image content to input image content preserved. A change in the
rotation about the z axis or angular orientation of incoming light
beams WI with respect to the x-y plane can cause a corresponding
symmetric change in rotation or angular orientation of outgoing
light from out-coupling diffractive optic (ODO) 120. From the
aspect of image orientation, turning grating TG is intended to
function as a type of optical relay, providing expansion along one
axis of the image that is input through the in-coupling diffractive
optic (IDO) 110 and redirected to out-coupling diffractive optic
(ODO) 120. Turning grating TG is typically a slanted or square
grating or, alternately, can be a blazed grating. Reflective
surfaces can alternately be used for turning the light toward the
out-coupling diffractive optic 120.
[0047] Beam expansion in two different dimensions is provided when
using the arrangement of FIG. 2. Turning grating TG expands the
diffracted beam from in-coupling diffractive optic 110 in the y
direction as shown. Out-coupling diffractive optic 120 further
expands the diffracted beam in the x direction, orthogonal to the y
direction as shown.
[0048] The known imaging light guide 20 that is shown in FIG. 2 has
been used in a number of existing head-mounted device (HMD) designs
for providing image content to a viewer. This type of beam expander
is particularly well-suited to augmented reality applications in
which image content can be superimposed on a real-world view as
seen through the transparent imaging light guide.
[0049] One acknowledged shortcoming of the known imaging light
guide beam expander relates to color quality. By design, a
diffraction grating is optimized for a particular wavelength, with
progressively degrading imaging performance as wavelengths deviate
further from the specified wavelength. Moreover, not only does
performance shift according to wavelength, but changes in incident
angle have more pronounced effects that vary with wavelength
differences. Because of this, undesirable color shifts can be
perceived across an image field when using the known type of
diffractive beam expander. The color shift problem proves extremely
difficult to compensate for in software, since the amount of color
shift can vary widely across the image field.
[0050] One approach for addressing the color shift problem is using
separate waveguides to serve the different primary color channels,
with diffraction elements suitably designed for handling light of
different wavelength bands. One proposed approach stacks of
multiple waveguides to effect beam expansion. Stacking can be used
to delegate the separate red (R), blue (B), and green (G) color
channels to individual waveguides, wherein the diffractive
components for each waveguide are designed suitably for light of
different wavelength bands. Cross-talk between color channels is
reduced using stacked waveguides with separate diffraction gratings
and optional color filters.
[0051] While stacking approaches can achieve some measure of
channel separation, the added weight, size, complexity, and cost of
stacked waveguide solutions can be significant. It can readily be
appreciated that solutions that would provide separate color
channels within a single waveguide, without appreciable color
channel crosstalk, would be advantageous for helping to reduce
color shifts and improve color quality overall.
Double-Sided Imaging Light Guide
[0052] FIG. 3A is a perspective view that shows an embodiment of an
imaging light guide 100 having two color channels C.sub.R and
C.sub.BG and formed on a single substrate. Color channels C.sub.R
and C.sub.BG can be centered at wavelengths that are at least 50 nm
apart, for example. Imaging light guide 100 is formed as a
double-sided diffractive beam expander, eliminating the need for
stacked waveguide solutions in order to reduce color channel
crosstalk. Image-bearing light for both color channels is incident
on an in-coupling diffractive optic 110.sub.BG that diffracts the
light of one of two color channels into the light guide 100.
[0053] FIG. 3B is an exploded view that shows an embodiment of an
imaging light guide 100 of FIG. 3A having two color channels and
formed on a single waveguide substrate S. The exploded view
visually separates the front and back surfaces F and Bk,
respectively, of the substrate S from each other. It must be
emphasized that there is only a single waveguide of substrate S;
each surface of the substrate S has the diffractive structures that
serve one of the two color channels. Components shown on the right
(front surface F) portion are primarily for one channel; components
shown on the left (back surface Bk) are for a second channel. In
the example shown, one color channel C.sub.BG is provided for green
and blue light (from about 450-550 nm); a second color channel
C.sub.R is provided for red light (from about 610-780 nm). Color
channel C.sub.BG has diffractive elements 110.sub.BG, 120.sub.BG
and TG.sub.BG formed on a coating that lies against the front
surface F of substrate S. Color channel C.sub.R has diffraction
elements 110.sub.R, 120.sub.R and TG.sub.R formed on a coating that
is applied onto the rear or back surface Bk of substrate S. For the
respective color channels, the in-coupling diffractive optics
110.sub.R and 110.sub.BG align with each other along a common
normal to the parallel front and back surfaces F and Bk. Similarly,
the out-coupling diffractive optics 120.sub.R and 120.sub.BG also
align along a common normal to the front and back surfaces F and
Bk. The respective turning gratings TG.sub.R, TG.sub.BG are not
similarly aligned.
[0054] It should be noted that any of a number of arrangements of
color channels and their associated bandwidth ranges can be used,
such as including green and red wavelength bands within one color
channel and blue wavelength bands in another color channel.
Cross-Talk Concerns
[0055] Cross-talk between color channels can be a problem with any
type of imaging system, including arrangements using multiple
stacked waveguides, but is a particular concern for designs using a
single waveguide. One approach for defeating crosstalk separates
the optical paths within the light guide as much as is possible,
both in terms of angle and of distance. For the example, as shown
in FIGS. 3A and 3B, the path of the red light in color channel
C.sub.R is separated from the path of the blue-green light in color
channel C.sub.BG by both angle and distance, so that "leakage" of
light to the wrong color path does not occur or is negligible.
Although this goal is straightforward, conventional methods for
achieving this goal have not been highly successful. Embodiments of
the present disclosure, however, provide methods for color channel
separation that make it possible to design and use a pupil expander
formed on a single substrate.
[0056] For a better understanding of the solutions proposed herein,
it is instructive to examine the behavior of different parts of the
optical system in light of the cross-talk prevention strategy
outlined above. FIG. 4A is a side view that shows the arrangement
and behavior of in-coupling diffractive optics 110.sub.BG and
110.sub.R. Blue/green light, shown by a dashed line BG, is
diffracted by in-coupling diffractive optic 110.sub.BG into the
waveguide substrate S and propagates within the substrate S via
TIR. A portion of this light reflects from in-coupling diffractive
optic 110.sub.R and also reflects from in-coupling diffractive
optic 110.sub.BG as it moves along the substrate. Red light,
indicated by a solid line R, transmits through in-coupling
diffractive optic 110.sub.BG and is diffracted, in reflection, by
in-coupling diffractive optic 110.sub.R for propagation within the
substrate S via TIR.
[0057] Because it is a side view, FIG. 4A cannot show the angular
difference between light diffracted from each in-coupling 110 in
the plane of substrate S. Returning for a moment to the perspective
view of FIG. 3B, it can be seen that the R and the BG light beams
trace different paths from in-couplings 110.sub.R and 110.sub.BG,
according to grating rotation. In FIGS. 4A and 4B, grating
rotation, corresponding to the angular distance between grating
vectors k.sub.R and K.sub.BG, is represented by a rotation angle
.PHI., shown in top view relative to the upper and lower
in-coupling diffractive optics 110.sub.BG and 110.sub.R in FIG. 4A
and in perspective view in FIG. 4B.
[0058] The grating direction, corresponding to the grating vectors
k.sub.R and k.sub.BG, determines the path of light that is
diffracted by each in-coupling diffractive optic 110.sub.R and
110.sub.BG. Peak separation between paths is achieved when the
paths of the R and BG light beams are orthogonal to each other;
this maximum path separation occurs when rotation angle .PHI. is at
or very near 90 degrees. As angle .PHI. decreases from 90 degrees,
entry of light into the wrong path and resulting cross-talk become
increasingly more likely.
[0059] In each color channel C.sub.BG and C.sub.R, the respective
turning grating TG.sub.BG and T.sub.GR redirect incident light from
the waveguide at a nominal 60 degree angle. The turning gratings
TG.sub.BG and T.sub.GR are designed and oriented specifically to
provide this behavior and generally operate to accept diffracted
light input and provide redirected light output at this
comparatively fixed angle.
[0060] Out-coupling diffractive optics 120.sub.BG and 120.sub.8
that face each other (formed on opposite surfaces along a common
normal) provide best performance with input light that is
orthogonal. Out-coupling diffractive optics 120.sub.BG and
120.sub.R then have their relative grating angles at orthogonal to
each other. At the out-coupling optics 120.sub.BG and 120.sub.R,
the likelihood of color channel cross-talk increases as grating
angles and incident angles diverge from orthogonal.
[0061] The geometric constraints on respective angles needed for
best performance of in-coupling and out-coupling diffraction
gratings, as outlined above, cannot be met without making at least
some type of compromise. The turning grating, for example, provides
some small degree of adjustability for turning angle .theta., based
on the pitch P.sub.new, which can be generally computed based on
the input pitch P.sub.input, using:
P.sub.new=P.sub.input(2 cos .theta.)
[0062] Thus, for turning light from an in-coupling diffractive
grating 110 having a 350 nm pitch, a turning grating TG having a
305 nm pitch would be required to provide a turning angle .theta.
of 55 degrees. Achieving a smaller turn angle would require a very
high pitch that might easily be too difficult or costly to
fabricate. For example, a 272 nm pitch grating would be required
for a turning angle .theta. of 50 degrees. Thus, although it can be
possible to adjust the turning angle by a few degrees, it is far
more practical to make some adjustment to the rotational angle
.PHI. between in-coupling diffractive optic 110 and out-coupling
120 diffractive optic, to angles less than the ideal orthogonal
angle.
[0063] Adjusting the gratings rotation angles so that components
fit within the conventional waveguide footprint and so that
rotation angles for facing diffraction gratings differ by the
largest possible angle yields the beam expander 140 design shown in
FIG. 5. Overlaying or facing gratings are shown as slightly offset
for clarity; in practice, facing in-coupling and out-coupling
optics are precisely aligned with each other, as described
previously. Here, angle .PHI. between gratings rotations for
in-coupling diffractive optics 110.sub.BG and 110.sub.R is 60
degrees. Out-couplings 120.sub.BG and 120.sub.R have their grating
vectors similarly rotated with respect to each other by 60 degrees
in the plane of the waveguide.
[0064] The arrangement shown in FIG. 5 is workable and provides a
two-channel solution on a single substrate. Color channel crosstalk
with this arrangement, however, is still clearly perceptible,
indicating that the color paths inadvertently "leak" into each
other even at the relatively high gratings rotation angles that are
used. The persistence of color crosstalk with the configuration of
FIG. 5 strongly suggests that further compromising the desired
angular rotations would be undesirable.
[0065] The graph of FIG. 6 shows characteristic behavior for
first-order reflective diffraction from a diffraction grating
designed for Red light with grating spacing slightly smaller than
red wavelengths, here about 510 nm. Diffraction efficiency is
plotted against rotational angle .PHI. for incident blue light at
475 nm at an approximately 40 degree incidence angle, as the light
beams would be traveling through the waveguide substrate with TIR.
This graph shows a general behavior characteristic that is used for
turning gratings TG. In addition to this, the graph of FIG. 6 also
suggests that one contributor to color channel cross-talk at
particular incidence angles may be the in-coupling diffractive
optics themselves.
[0066] When the diffraction gratings of in-coupling diffractive
optics 110.sub.BG and 110.sub.R are rotated so that angle .PHI. is
below about 40 degrees, diffraction efficiency is less than 20% so
that very little first-order diffraction of light at 40 degree
angular incidence occurs. When this is the case, blue/green
incident light beams at 40 degrees can simply reflect from the Red
in-coupling 110.sub.R grating surface, as in conventional TIR. As
rotation angle .PHI. increases above 50 degrees, however, first
order reflective diffraction increases dramatically. At 60 degrees,
first-order reflective diffraction approaches a maximum, at nearly
75% for a target wavelength.
[0067] Notably, FIG. 6 shows behavior for light incident at TIR
angles (exceeding 40 degree incidence). As noted above, this same
effect is used for design of a turning grating TG. However, an
undesirable turning grating effect can also occur in the
in-coupling region of the waveguide, between in-coupling
diffractive optics 110.sub.R and 110.sub.BG. Where this unintended
effect occurs, it may cause one or the other in-coupling
diffractive optics to behave as a turning grating for light from
the opposite in-coupling diffractive optic instead of allowing TIR.
At efficiencies close to 80%, the opposing in-coupling then begins
diverting light from its intended TIR path and re-directing some of
the diverted light into the path intended for the opposite color
channel. That is, red light is now inadvertently coupled into the
blue-green light path and vice-versa.
[0068] FIG. 7 shows this problem schematically for blue-green light
BG. The intended path of this BG light is shown in solid line. The
path for red light is omitted for clarity. A second BG light path
144, shown in dashed line format, indicates that some of the BG
light that should be reflected from in-coupling diffractive optic
110.sub.R has now been diffracted instead, and unintentionally
diverted to the red light path. Thus, with in-coupling diffractive
optics 110.sub.R and 110.sub.BG at a relatively high rotation
angle, e.g., .PHI.=60 degrees, a high degree of color crosstalk can
be observed. This same effect can be true also for Red light that
is incident on the in-coupling diffractive optic 110.sub.BG,
effectively diverting some Red light to the blue-green path.
[0069] Thus, from what is shown in FIG. 7, it can be appreciated
that one source of color channel crosstalk that can prove to be
particularly troublesome is light interaction at the in-coupling
diffractive optics 110.sub.R and 110.sub.BG. Some of the light
incident on each diffraction grating is inadvertently diffracted
into the opposite color channel. Providing proper rotation of the
respective input/output gratings can help to reduce the crosstalk
problem but can also present constraints that can be difficult or
unworkable in some cases. Embodiments of the present disclosure
address the problem of cross-talk resulting from light interaction
between the in-coupling diffractive optics 110.sub.R and 110.sub.BG
by constructing the imaging light guide with dichroic coatings
whose angular characteristics are tuned to reflect and transmit
light differently according to both wavelength and incidence angle.
Light of the same wavelength may be selectively transmitted or
reflected according to its incident angle.
[0070] The perspective view of FIG. 8A and cross-sectional view of
FIG. 8B show a double-sided imaging light guide 200 that is formed
as a multilayer structure having a waveguide substrate 204 and
opposing top and bottom coating layers 206 and 208, respectively.
Layers 206 and 208 can be formed from an optical polymer, for
example. The various in-coupling 110, out-coupling 120, and turning
grating TG components for each of the two color channels are formed
on different surfaces of the multilayer structure. Sandwiched
between the facing in-couplings that are formed on the top and
bottom coating layers 206 and 208 are dichroic patches 210.sub.BG,
and 210.sub.R, configured for handling light in the different color
channels C.sub.BG and C.sub.R. Each dichroic patch 210.sub.BG and
210.sub.R extends over a small portion of the substrate surface.
Each dichroic patch 210.sub.BG and 210.sub.R is sized to extend
over the area of the surface that lies between its corresponding
in-coupling diffractive optic 110.sub.R or 110.sub.BG and waveguide
substrate layer 204. The dichroic patches 210.sub.BG and 210.sub.R
are opaque, so that extending their size beyond the area of the
in-coupling optics would tend to obstruct the field of view.
[0071] The side view of FIG. 9A shows a filter characteristic for
dichroic patches 210.sub.BG and 210.sub.R handling light of the red
color channel C.sub.R.
[0072] FIG. 9B presents a filter characteristic curve showing
dichroic filter transmittance with respect to incident light for
first dichroic patch 210.sub.BG. At less than 15 degrees incidence,
the red light is transmitted through dichroic patch 210.sub.BG. At
greater than 40 degrees incidence, however, as indicated by a
dashed line, the red light is reflected from dichroic patch 210BG.
Blue-green light is always transmitted.
[0073] FIG. 9C presents a filter characteristic curve that shows
transmittance behavior of the dichroic coating of second dichroic
patch 210.sub.R. Red light incident at any angle from 0 to 40
degrees always transmits through dichroic patch 210.sub.R.
Blue-green light is always reflected.
[0074] The side view of FIG. 10 shows how dichroic patches
210.sub.6G and 210.sub.R handle light of the blue-green color
channel C.sub.BG. Blue-green light incident at any angle always
reflects from dichroic patch 210.sub.R. Blue-green light transmits
through dichroic patch 210.sub.6G.
[0075] FIG. 11 combines the results provided by dichroic patches
210.sub.BG and 210.sub.R at the in-couplings. The positions of
dichroic patches 210.sub.BG and 210.sub.R could be reversed with
respect to the incident light, so that the incident light first
encounters dichroic patch 210.sub.R, for example. This would
require corresponding changes in dichroic coating characteristics
and color channel component placement using the basic arrangement
described for the example of FIG. 11.
[0076] The perspective view of FIG. 12 shows a display system 60
for three-dimensional (3-D) augmented reality viewing using imaging
light guides of the present disclosure. Display system 60 is shown
as an HMD with a left-eye optical system 54l having a beam expander
140l for the left eye and a corresponding right-eye optical system
54r having a beam expander 140r for the right eye. An image source
52, such as a picoprojector or similar device, can be provided,
energizable to generate a separate image for each eye, formed as a
virtual image with the needed image orientation for upright image
display. The images that are generated can be a stereoscopic pair
of images for 3-D viewing. The virtual image that is formed by the
optical system can appear to be superimposed or overlaid onto the
real-world scene content seen by the viewer. Additional components
familiar to those skilled in the augmented reality visualization
arts, such as one or more cameras mounted on the frame of the HMD
for viewing scene content or viewer gaze tracking, can also be
provided.
Imaging Light Guide Fabrication
[0077] Various processes can be used to fabricate and assemble the
imaging light guide, as shown in the exploded view of FIG. 8A.
[0078] Dichroic filters are a type of thin-film interference
filter, which are treated or formed to provide a
wavelength-selective filter characteristic as a result of the
interference effects that take place between incident and reflected
waves at boundaries between interleaved layers of materials having
different refractive indices. Interference filters conventionally
include a dielectric stack composed of multiple alternating layers
of two or more dielectric materials having different refractive
indices. In a conventional thin-film interference filter, each of
the respective interleaved layers of the filter stack deposited on
the substrate is very thin, e.g., having an optical thickness
(physical thickness times the refractive index of the layer) on the
order of one-quarter wavelength of light. A filter having a filter
characteristic with reflection of at least one band of wavelengths
and transmission of at least a second band of wavelengths
immediately adjacent to the first band, such that the filter
enables separation of the two bands of wavelengths by redirecting
the reflected band, is conventionally called a "dichroic"
filter.
[0079] Optical filters formed or configured according to
embodiments of the present disclosure can generally employ the
basic structure of a thin film interference filter. In this basic
structure, a plurality of extremely thin discrete layers of
material are deposited onto a surface of a substrate in some
alternating or otherwise interleaved pattern as a filter stack,
wherein the optical index between individual layers in the filter
stack changes abruptly, rather than continuously or gradually. The
plurality of layers include at least a number of first layers
having a first refractive index n.sub.L interleaved with a number
of second layers having a second refractive index n.sub.H that is
greater than the first refractive index. One or more additional
layers having refractive indices not equal to either n.sub.H or
n.sub.L can also be in the filter stack. In conventional thin film
designs, two discrete layers are alternated, formed with
thicknesses very near the quarter-wavelength thickness of some
fundamental wavelength. The addition of a third material or other
additional materials in the thin film stack helps to fine-tune
filter response. The numerical differences between the index of
refraction in the high and low index of refraction materials
affects the number of thin film layers required for forming a
filter with a particular transmittance characteristic. Where the
difference between the indices of refraction of the high and low
index materials is large enough, fewer alternating layers are
needed for achieving the same transmittance (density) values.
[0080] A wide variety of materials can be used to form the
plurality of discrete material layers in the filter stack. Among
such materials, non-limiting mention is made of metals, metallic
and non-metallic oxides, transparent polymeric materials, and
so-called "soft" coatings, such as sodium aluminum fluoride
(Na.sub.3AlF.sub.6) and zinc sulfide (ZnS). Further non-limiting
mention is made of metallic oxides chosen from silicon dioxide
(SiO.sub.2), tantalum pentoxide (Ta.sub.2O.sub.5), niobium
pentoxide (Nb.sub.2O.sub.5), hafnium dioxide (HfO.sub.2), titanium
dioxide (TiO.sub.2), and aluminum pentoxide (Al.sub.2O.sub.5). The
interleaved material layers may include at least two distinct
materials. As a non-limiting example, the filters according to the
present disclosure can include a plurality of distinct alternating
Nb.sub.2O.sub.5 and SiO.sub.2 layers, which have indices of
refraction of 2.3 and 1.5, respectively. Alternatively, filters in
accordance with the present disclosure may use an interleaved
pattern with at least three distinct materials, such as distinct
Nb.sub.2O.sub.5, SiO.sub.2, and Ta.sub.2O.sub.5 layers, each layer
having a characteristic index of refraction. Of course, more than
three materials and other combinations of materials may also be
used within the interleaved layer pattern.
[0081] Generally, the filters in accordance with the present
disclosure can be manufactured using deposition methods and
techniques that are well known to those skilled in the optical
coatings art. For example, these filters may be made with a
computer controlled ion beam sputtering system, capable of
depositing a plurality of discrete alternating material layers,
wherein the thickness of each layer may be precisely
controlled.
[0082] Referring back to FIG. 8A and particularly to the depicted
coatings 206 and 208, an embodiment of the present disclosure uses
UV embossing to form each thin polymer layer that extends over at
least a portion of the top and bottom surfaces of the waveguide
substrate layer 204. To form the diffractive optics, a quartz mold
having an etched pattern can be used to transfer its patterning
onto to the polymer as the polymer sets under UV light.
Alternately, the diffraction components can be formed on one or
both outer surfaces of the waveguide guide substrate S using
nano-imprinting methods, for example. The coatings 206 and 208 can
also be formed as or applied as films.
[0083] In-coupling diffractive optics 110 and out-coupling
diffractive optics 120 can be diffraction gratings or formed as
volume holograms, or formed from a holographic polymer dispersed
liquid crystal, for example. At least one of the in-coupling and
out-coupling diffractive optics can be a surface relief diffraction
grating. The waveguide substrate of the imaging light guide is a
transparent optical material, typically glass or optical polymer
material with sufficient index of refraction for supporting TIR
transmission between in-coupling diffractive optic, turning
grating, and out-coupling diffractive optic.
[0084] In-coupling diffractive optics 110, turning gratings TG, and
out-coupling diffractive optics 120 have different grating periods
appropriate to their respective color channels. Typically the
grating pitch, or grating period, is a value from 75 to about 90
percent of the central wavelength for a color channel. For example,
the in-coupling diffractive optic 110.sub.R for the red channel
(620-670 nm), in an exemplary embodiment, has a period of 510 nm, a
depth of 205 nm, 50/50 fill, and a 45-degree slant.
[0085] The invention has been described in detail with particular
reference to a presently preferred embodiment, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention. The presently disclosed
embodiments are therefore considered in all respects to be
illustrative and not restrictive. The scope of the invention is
indicated by the appended claims, and all changes that come within
the meaning and range of equivalents thereof are intended to be
embraced therein.
* * * * *